The present invention relates to automated optical inspection and more particularly to pre-processing of an analog image signal prior to determining digitizing thresholds.
In automated optical inspection applications which entail an analog video source and some form of digital image processing, the analog video signal representing the image source must be converted to digital form by some type of analog-to-digital conversion process. One limitation of the efficiency of this process is determined by the amount of background level in the analog signal as a result of the reflectivity and color of the background of the image source. The reflectivity and color of the background may also vary across any given image source. The "white" and "black" levels of the analog signal correspond to the reflective and non-reflective surface areas of the image plane. This background component, referred to as the "black" level when the "white" level represents the pertinent information, can introduce inaccuracies in the digital signal resulting in a reduced "process window" limiting the defects which can be detected and can result in the need to alter the digitizing sensitivity which requires operator intervention.
Previous techniques of dealing with background levels are 1) determining a minimum digitizing sensitivity below which will be assumed and processed as background, or 2) by using a peak detecting process which is an averaging function rather than a real time or nulling function. These peak values were then used to set a digitizing threshold rather than alter the video signal.
It is an object of the present invention to provide analog video processing method and apparatus which enhances the signal-to-noise ratio of the analog signal and therefore increase the digitizing sensitivity.
It is an object of the present invention to provide analog video processing method and apparatus suitable for use in detecting defects in a wide variety of printed circuit boards including those having multiple layers.